The present invention relates to a method of coating a contactor of a circuit breaker or the like with a noble metal and particularly relates to the improvement of the process of applying the coating.
The contactor of a conventional circuit breaker is made of a copper member plated with silver not by a laser beam but by wet plating.
A method of coating with a metal other than a noble metal by a laser beam is disclosed in Japanese Patent Application Laid-Open No. 57-155363. FIG. 4 shows a device for coating with a metal by using this method. FIG. 4 indicates a laser oscillator 1, bending mirrors 2a, 2b and 2c which change the direction of a laser beam 1a emitted from the laser oscillator and constitute an optical system for transmitting the laser beam, a condensation lens 3 for condensing the laser beam, a powder feed means 4 for feeding a powder of the coating metal, a powder feeding pipe 5 for conveying the powder, a nozzle 6 for jetting the powder onto an underlying metal 7, a coating metal powder 8 jetted from the nozzle 6, a metal film 9 coated on the underlying metal 7, an arrow A indicating the travelling direction of the laser beam, and another arrow B indicating the travelling direction of the underlying metal.
The conventional device for coating the underlying metal 7 with the metal film 9 is constructed as described above. The laser beam 1a emitted from the laser oscillator 1 is transmitted by the bending mirrors 2a, 2b and 2c as shown by the arrow A. At the same time, the underlying metal 7 is moved as shown by the arrow B, and the coating metal powder 8 is fed from the nozzle onto the underlying metal 7. The laser beam 1a condensed to an appropriate diameter by the condensation lens 3 is irradiated upon the coating metal powder 8 so as to be absorbed by the powder and the surface layer of the underlying metal 7 to melt them to metallically bond them to each other. The underlying metal 7 is thus coated with the metal film 9.
Usually, the underlying metal 7 is an iron alloy and the coating metal 8 is nickel, chromium or an alloy thereof in the above-mentioned conventional method. In such a case, it is relatively easy to make a metal film 9 having good properties.
When an underlying metal made of a material such as a copper or an aluminum alloy is coated with a noble metal such as gold or silver, some differences exist as compared with the above-mentioned case in that the underlying metal is more likely to be oxidized, the laser beam absorption ratio of copper, silver and gold is much lower than that of iron and the like, and the thermal conductivity of the underlying metal is high.
FIG. 5 shows a sectional view of a noble-metal-coated portion in which an underlying metal 7 such as copper and an aluminum alloy is coated with a noble metal such as gold or silver in accordance with the above-described conventional method. As shown in FIG. 5, the laser-beam-irradiated portion has an oxide film 11b harder and thicker than that of a portion 11a surrounding the laser-beam-irradiated portion. Since the thermal conductivity of the underlying metal 7 is high and the laser beam absorption ratio of the noble metal is low, the temperature of the surface layer of the underlying metal 7 does not easily reach a bonding temperature and the powder of the noble metal on the laser-beam-irradiated portion is molten and condensed to make separate particle-like materials 12 so that the noble metal does not become a uniformly coated film.